The present invention relates to an image processing apparatus, and more particularly, to an image processing apparatus which can produce a half tone image from multi-level image data input thereto.
In recent years, a laser beam printer making use of electrophotographic process is attracting attention as one type of printers which can operate at a high speed with reduced noise. Such a laser beam printer is typically used for the purpose of recording data such as characters and line patterns. The images of the characters and line patterns are so-called binary images which can be expressed by two states: namely, black and white. Since reproduction of half-tone image is unnecessary, the construction of the printer can be simplified.
Methods are known which reproduce quasi-half tone image with a binary recording apparatus, such as the dither method and density pattern method. As well known to those skilled in the are however, the dither method and density pattern method are disadvantageous in that they cannot produce images with high resolution. Under this circumstance, a printer has been recently developed in which a semiconductor laser is driven by a pulse-width-modulated (PWM) image signal so that half-tone image can be formed even by binary recording methods. According to this PWM method, it is possible to obtain a print output with high degrees of resolution and gradation. In particular, this printing technique has become indispensable in color image printing apparatus.
Laser beam printers relying upon PWM methods, however, encounter with various problems peculiar to this type of printer. One of these problems pertains to lack of stability of the density in the printing of image. This problem is inherent in electrophotography. The other problem is encountered when a semiconductor laser is driven through pulse width modulation.
These problems will be discussed in more detail.
FIG. 11 shows the general construction of a printer portion used in an electrophotographic system. The printer portion has a photosensitive drum 301 adapted to be rotated in the direction of the arrow about the axis of a shaft 306, components arranged around the photosensitive drum 301 such as a charger 302, a developing unit 303, a transfer charger 304 and a cleaning device 305, and an optical system arranged at the upper side of the photosensitive drum 301 as viewed in the drawings.
The optical system includes a semiconductor laser unit 306, a polygonal mirror which rotates at a constant high speed, an f-.theta. lens 308, a light-shielding plate, and so forth. Time-serial digital pixel signals computed and output from an image reader or an electronic computer (not shown) are PWM modulated and delivered to the semiconductor laser unit 306. The semiconductor laser unit 306 turns on and off the generation of a laser beam in accordance with the levels of the PWM-modulated pixel signals and directs the beam towards the polygonal mirror 307. Since the polygon mirror 307 is rotating at a high constant speed, the laser beam applied to one side of the polygonal mirror 307 is reflected in an oscillatory manner so as to scan and expose the portion of the photosensitive drum between the charger 302 and the developing unit 303, from the proximal end to the distal end as viewed in the drawings.
In general, the photosensitive drum 301 exhibits changes in the exposure sensitivity and residual potential, due to a change in the environmental condition and elapse of time. In addition, the developing material such as a toner used in the developing unit 303 exhibits a large fluctuation in the developing density according to a change in the amount of charges. This problem, i.e., lack of stability of the image density, is a problem inherently possessed by electrophotographic technique itself, but significantly affects formation of low-density image by a PWM type laser printer.
FIG. 9 is a circuit diagram of a PWM circuit proposed by the assignors, while FIG. 10 is a circuit diagram showing a laser driver circuit. FIG. 12 is a timing chart illustrative of the operation of the PWM circuit.
Referring to FIG. 9, the PWM circuit includes a TTL latch circuit 401 for latching 8-bit pixel signals, a level converter 402 for converting a TTL logical level to a high-speed ECL logical level, an ECL D/A converter 403, an ECL comparator 404 for generating a PWM signal, a level converter 405 for converting an ECL logical level to a TTL logical level, a clock generator 406 for generating a clock signal 2f of a frequency which is twice as high as the pixel clock signal f, a triangular wave generator 407 for generating substantially ideal triangular wave signals, and a 1/2 frequency dividing circuit for conducting a 1/2 frequency division of the clock signal 2f. In order to enable the circuit to operate at a high speed, ECL logical circuits are arranged everywhere in the circuit.
The operation of this circuit will be explained with reference to FIG. 12.
In these Figures, signals (a) and (b) represent, respectively, the clock signal 2f and the pixel clock signal f having a period which is twice as large that of the clock signal 2f. In the triangular wave generator 40 also, a triangular wave signal (c) is generated after a 1/2 frequency-division of the clock signal 2f, in order to maintain the duty ratio of the triangular wave signal at 50%. Furthermore, the triangular wave signal (c) is converted to an ECL level (0 to -1 V) so as to form a triangular wave signal (d).
Meanwhile, the pixel signal latched by the latch circuit 401 is variable over 256 gradation levels between 00H (white) to FFH (black). The symbol "H" represents a hexadecimal notation code. The pixel signal (e) represents ECL voltage levels as obtained through a D/A conversion of a plurality of pixel signal values by a D/A converter 403. For instance, the pixel signal for the first pixel has a voltage of black pixel level FFH, the pixel signal for the second pixel has a voltage of a half tone level of 80 H, the pixel signal for the third pixel has a voltage of a half tone level of 40 H and the pixel signal for the fourth pixel has a voltage of a half tone level 20 H. The comparator 404 is adapted to produce, through a comparison between the triangular wave signal (d) and the pixel signal (e), PWM signals such as pulse widths T, t.sub.2, t.sub.3 and t.sub.4. The PWM signal is then converted to a TTL level of 0 V or 5 V so as to become a PWM signal (f) which i delivered to a laser driver circuit 500.
FIG. 10 shows the laser driver circuit which is of a constant current type, and the semiconductor laser device 501. This semiconductor laser device 501 emits a laser beam when the switching transistor 502 is on and terminates the emission when the switching transistor is turned off. The switching transistor 502 cooperated with a transistor 504 in forming a transistor pair which in turn forms a current switching circuit capable of controlling on/off (conversion) of the constant current which is to be supplied to the semiconductor laser device 501, in accordance with the PWM signal input thereto. This constant current is supplied from a constant current source transistor 505 and can be varied. The input laser power value input thereto is converted into an analog voltage by a D/A converter 503 and is compared with a reference voltage. The level of the constant current is determined in accordance with the result of the comparison.
However, the following problem is still encountered even when the above-described control is conducted, due to response characteristics of the semiconductor laser device 501. Referring to FIG. 12, representing the maximum emission time per pixel by T (sec), a change in the pulse width between 0 and T (sec) theoretically should cause the semiconductor laser device 501 to emit the beam over a time which corresponds to the pulse width. Actually, however, a signal waveform (g) for driving the laser device is different from the PWM signal (f) due to the fact that the PWM signal (f) is transmitted through the semiconductor laser device 501 and the driving circuit 501, with the result that a delay is caused in the turning on and off of the laser beam. This delay does not cause any problem when the pulse width is T or t.sub.2. However, when the pulse width is t.sub.3, the signal for driving the semiconductor laser device cannot be completely switched to ON state. When the pulse width is t.sub.4, the semiconductor laser device 501 fails even to operate materially. A beam effect (h) two-dimensionally illustrates the state of emission of the laser beam. The first pixel is completely black, so that the laser beam is kept on whole through one pixel period. However, when the pulse width of the PWM signal is extremely short as, for example, t.sub.3 =10 ns, the state of generation of the laser beam is too unstable to form an image by electrophotographic process, not to mention a problem as to whether the laser beam is actually generated. In such a case, stable formation of density can no more be expected. Thus, in the gradation expression according to the PWM method, there is a practical limit in the minimum pulse width which can form an appreciable density. If this limit is t3=10 ns for example, gradation is always white whenever the pulse width is below this lower limit of 10 ns, i.e., in the highlight portion.